1. Field of the Invention
This invention relates generally to the field of seismic geophysical prospecting and more particularly to the sequential detonation of a plurality of lump explosive charges located at spaced intervals within a bore hole.
2. Prior Art
The detonation of distributed charges within a bore hole in the practice of seismic surveying through what is termed "sequential shooting" is well known. Distributed charges may assume the form of an elongated explosive cord or alternatively, they may consist of lump explosive charges at spaced intervals. This invention is concerned with the latter type.
In this art, the concept is to fire the lump explosive charges at intervals such that the associated seismic wave fronts reinforce each other in a given direction, normally down the shot hole. To do this, the array of charges is detonated from the top charge downwardly and the timing between successive detonations is made equal to the time of travel of the seismic waves along the formation between successive charges.
One form of apparatus for carrying out this technique relies upon sequentially energizing a series of pressure actuated switches in response to the arrival of a downwardly advancing seismic wave front produced by the detonation of the first charge. As each switch is energized, a detonator or blasting cap is set off so as to fire the associated explosive charge. Such an apparatus is described in U.S. Pat. No. 3,196,974 issued on July 27, 1965 to V. M. Barnes. According to the Barnes patent, each charge detonating circuit or "phaser" is activated by the operation of a mechanical switch. The switch incorporates a metallic diaphragm which moves responsive to a seismic wave front so as to contact a centrally located member to produce switch closure. Each pressure responsive switch is located at a predetermined distance away from the explosive charge to be detonated by its closure. This distance is designed to compensate for the time it takes for switch closure to occur after the arrival of the seismic wave front. In this way, in accordance with the patent, the pressure wave will not have traveled past the charge to be detonated before this event occurs. Thus, reinforcement is said to be achieved between each seismic pressure wave front and the succeeding one.
A disadvantage of devices of this character is that they are dependent upon formation seismic velocity. In order to accurately position the pressure sensor in relation to the associated detonator, the formation accoustic velocity governing the advancement of the seismic wave front from charge to charge must be accurately known. Furthermore, this spacing must be varied to accomodate the fact that differing formations will have differing seismic velocities. In addition, the very fact that there must be some significant spacing between the pressure sensor and the detonator imposes a limitation on the design and physical character of the apparatus. Beyond the above problems is the further complication that the behavior of seismic shock waves in the "near field", i.e., the immediate vicinity of an explosion, is difficult to interpret and analyze precisely as to wave form and velocity. These near field shock wave characteristics are typically quite different from the relatively stable acoustic behavior of the formation when measured remote from the source in what is termed the "far field". For this reason, assumptions concerning formation velocity should be regarded with considerable skepticism as applied to the design and operation of distributed charge apparatus.
Since the requisite time delay between successive distributed charges is likely to be on the order of a few micro-seconds, a further disadvantage of the use of mechanical switches in the context described above is their inherent lack of precise repeatability. This is due to metal fatigue, temperature and humidity effects, bore hole depth and other environmental factors.